The following description of the background of the invention is provided simply as an aid in understanding the invention and is not admitted to describe or constitute prior art to the invention.
In the semiconductor field, films formed by vacuum evaporation, vapour deposition, spin coating or dip coating are commonly used at various stages of the semiconductor fabrication process. Control and monitoring of the actual thickness and physical properties of thin film layers is absolutely essential to the function of the devices created using this technology. These characteristics must often be monitored during and after fabrication. “Optical thin film determination” refers to methods for determining the thicknesss of one or multiple thin layer(s) formed on a substrate surface. Such “thin films” range from about 1 nm to about 100 μm in thickness.
Typically, optical thin film measurements rely on changes in one or more characteristics of light reflected from a substrate comprising an “optical thin film test surface.” By this is meant that the surface is reflective of incident light, and is configured and arranged by selection of refractive index (n) and absorption coefficient (k) for generation of a signal directly due to a change in mass or thickness upon the surface. The signal is obtained by illuminating the surface with light; light is reflected from the surface or transmitted through the surface, and any thin film upon the surface will alter the color, ellipticity, and/or intensity of one or more wavelengths in the reflected or transmitted light due to an interference effect. This extent of the alteration, and hence the signal obtained, depends on the mass or thickness of any surface film(s).
Devices for optical measurement of thin films generally fall into two instrument classes: reflectometers and ellipsometers. Reflectometry is based upon measurement of changes in intensity and/or color of light reflected from the optical thin film test surface; ellipsometry is based on measurement of changes of the polarisation of light reflected from the optical thin film test surface. Such methods are well known in the art. See, e.g., Tompkins and McGahan, Spectroscopic Ellipsometry and Reflectometry: A User's Guide, John Wiley and Sons, 1999, which discusses the nature of optical constants of materials, instrumental aspects of reflectometers, ellipsometric spectra, and single-wavelength ellipsometry, as well as analytical approaches for collecting and analyzing ellipsometric and reflectance data.
Because of the ability of such methods and devices to sensitively detect changes in film thickness at molecular dimensions, the application of optical thin film measurements to biological systems has become well established. For example, devices and methods for direct detection of binding reactions (e.g., in immunoassay, nucleic acid hybridization, etc.) has been described. See, e.g., U.S. Pat. Nos. 6,483,585; 6,355,429; 6,287,783; 6,060,237; 5,955,377; 5,639,671; 5,631,171; 5,629,214; 5,552,272; 5,550,063; 5,494,829. While such methods do not depend upon the presence of a signal development element (e.g., a fluorometric, luminescent, or calorimetric moiety) for production of a signal, amplification methods (e.g., the catalytic production of a precipitate or the binding of particles such as latex, gold, etc.) to provide additional mass or optical thickness may be employed to enhance detection of the binding reaction.